394 Herbert L. Haivkins — Studies on the Echinoidea. 



structure is composed of a varying number of paired interambulacrals 

 at its base, and has an apparently undivided plate along its free 

 margin (see Duncan, 1885). It is possible that this unpaired 

 plate may represent the primordial interambulacral (which is absent 

 from the peristomial margin in these forms) ; or perhaps an out- 

 growth from it prior to its resorption. But in Discoides, where the 

 primordial plate is situated on the peristomial margin, the distal 

 part of the ridge is definitely composed of paired plates. 



In Plesiechinus the only representative of the ridge is found as 

 a semilunar inward prominence of the primordial interambulacral. 

 It is, as far as my observations indicate, and as would be expected 

 from its position, absolutely undivided by sutures of any kind. In 

 Discoides the small median peristomial plate of the ridge is slightly, 

 but definitely, more prominent than the rest of the structure. It is, 

 almost certainly, the inner surface of the primordial plate. This 

 small portion of the ridge must therefore be the homologue of the 

 " ridge " of Plesiechinus. 



In the Cidaroida and Diademoida the ridge is only connected to 

 the normal coronal plates at its base, projecting almost vertically 

 from their horizontal inner surfaces. In Discoides, on the contrary, 

 the whole ridge, with the exception of an almost negligible rim at 

 its distal extremity, is recumbent upon the interambulacrum, which 

 is thickened so as to project in some cases (in internal aspect) to 

 a greater height than the ridge itself attains. (See PI. XXY, Fig. 3.) 

 The degree to which the ridge extends above its supporting plates 

 varies considerably, apparently with age. In some forms, where 

 the radiating buttresses are low and feeble, the projection of the 

 ridge may amount to nearly half its total height. It will be noticed 

 in Loven's figure (my PL XXV, Fig. 2) that the median suture of 

 the ridge is directly continuous with that of the normal part of the 

 interambulacrum. These distal plates of the ridge of Discoides, 

 which constitute the greater part of the ridge, surely represent the 

 inner surfaces of the paired interambulacrals next to the primordial, 

 modified as supports, perhaps as actual "slides", for the inclined 

 lantern. The projecting rim of the " ridge" need cause no surprise 

 when the extraordinary pillar-like and carinate internal develop- 

 ments of the inner surface of the Clypeastroida, and indeed of 

 Discoides itself, are taken into account. 



If this interpretation of the girdle is correct, it follows that the 

 girdle proper consists of the paired processes and small triangular 

 peristomial plate of the interambulacrum ; and that the bulk of the 

 ridge is merely a specialized portion of the thickened paired plates. 

 There is then no difficulty in considering the two small lateral plates 

 of Loven's figure, which form the interradial sides of the branchial 

 incisions, as the first paired columnals ; while the two large upper 

 plates represent the somewhat expanded and modified inner surfaces 

 of the second pair. All of the " ridge " except that part built of the 

 small proximal median plate, is then not homologous with the ridge 

 of a Diademoid. I would call it & "false ridge". It serves, like 

 the buttresses recently described in Plesiechinus, as a mechanical 

 support for the processes; and has the additional function of 



